1. Field of the Invention
The present invention is related to a process for preparing a catalyst composition for ethylene polymerization or copolymerization, in particular to a Ziegler-Natta catalyst system. The catalyst system of the invention is heterogenous and comprises at least one transition metal compound, at least one magnesium compound and at least one electron donor.
2. Description of the Prior Art
Catalysts of the Ziegler-Natta type are known for the production of highly isotactic polypropylene resins and are commercially available from e.g. Union Carbide Corporation. They are described in a number of U.S. patents, namely U.S. Pat. Nos. 4,414,132, 4,329,253, 4,657,995, 4,710,482, 4,804,648, 4,414,132, 4,329,253, 4,393,182, 4,497,905, 4,548,915, 4,520,163, 4,543,389, 4,477,639, 4,535,068, 4,657,995, 4,563,512, 4,663,299, 4,710,482, 4,804,648, 4,806,696 and 4,851,499.
When preparing this type of catalyst to produce highly isotactic polypropylene, nearly 27 kg of effluent for each kg of polypropylene catalyst produced is wasted in the catalyst production process. The waste consists mainly of titanium tetrachloride (or other transition metal salts), monochloro benzene, isopentane, magnesium chloride and other chemical impurities such as TiCly(EtO)x compounds. The effluent waste may generally comprise from about 10 to about 20 wt.-% transition metal, from about 20 to about 45 wt.-% monochloro benzene, from about 0 to about 5 wt.-% isopentane, from about 45 to about 65 wt.-% chlorine (as chlorides) and other chemical impurities such as from about 1 to about 10 wt.-% ethoxy compounds, from about 0 to about 2 wt.-% electron donor and from about 0.5 to 5 wt.-% solids. The effluent cannot be used again to prepare another batch of polypropylene catalysts unless the impurities are removed to a high extent. Distillation as described in U.S. Pat. No. 5,242,549 may be used as a means of purifying the effluent from its impurities. However, the costs of using the technology available today for recovering the effluent is significantly high and not justified for small size plants.
Today, the generated catalyst effluent waste is collected and disposed off. Disposing such a waste, however, is cumbersome and a costly process.
Therefore, it is an object of the present invention to find a beneficial way of re-using the effluent waste from a process for production of polypropylene catalysts of the Ziegler-Natta type.
This object is solved with a process for preparing a catalyst composition for ethylene polymerization or copolymerization, with the steps of:    (a) treating at least one magnesium alkoxide compound with the effluent waste from a process for production of a polypropylene catalyst of the Ziegler-Natta type, the effluent waste being mainly composed of about 10 to about 20 wt.-% transition metal based compounds, about 20 to about 50 wt.-% solvents, about 0 to about 2 wt.-% electron donors, and impurities;    (b) washing the product of step (a) with an inert hydrocarbon solvent; and    (c) drying the material for use as the catalyst composition.
Preferably, the transition metal based compounds comprise titanium tetrachloride, and the amount of titanium tetrachloride in the catalyst composition is preferably in the range of 1 to 10 wt. %, most preferably 2.5 to 3.5 wt. %.
In a preferred embodiment of the invention, the titanium loading of the catalyst composition is adjusted by adding excess electron donor to the effluent before adding to the magnesium alkoxide compound.
Preferably, the magnesium alkoxide compound comprises magnesium ethoxide, wherein the ethoxide content in the catalyst composition is preferably in the range of 1 to 9 wt. %, most preferably 2 to 5 wt. %, whereas the magnesium content in the catalyst composition is preferably in the range of 5 to 25 wt. %, most preferably 15 to 20 wt. %.
Preferably, the electron donor is a compound selected from the group consisting of ethyl benzoate and diisobutylphthalate, wherein the content of electron donor in the produced catalyst composition is preferably in the range of 0.01 to 20 wt. %, most preferably 10 to 15 wt. %.
The invention is also directed to processes for polymerization of ethylene to a high molecular weight polyethylene or for copolymerization of ethylene with a linear alpha-olefin using the catalyst composition according to the present invention.
Surprisingly, we have discovered that the effluent waste from a process for the production of polypropylene catalysts of the Ziegler-Natta type can effectively be used to manufacture a series of superactive catalysts for the polymerization or copolymerization of ethylene, without the need to purify the effluent and reclaim the essential products from it. No new equipment or retrofitting is needed to prepare the catalysts using the effluent. The catalysts can be prepared in the same plant facilities, which are used to prepare the prior art catalysts. The preparation of these new catalysts in this invention utilizes the prior art effluent which certainly will have a tremendous impact on the catalyst manufacturing efficiency for both polyethylene and polypropylene production.
The new catalysts produced from the crude effluent were examined closely in the lab and showed highly activity performance.